There are conventionally known inorganic fiber molded bodies produced by subjecting a slurry comprising inorganic fibers such as alumina fibers and silica fibers, inorganic particles, an inorganic binder, an organic binder and the like to dehydration molding process and then firing the resulting dehydration-molded product. The inorganic fiber molded bodies have been used as a refractory insulating material for high-temperature industrial furnaces because they have a relatively light weight, an easy-processing ability, a shape-retaining capability, and an excellent heat insulating property. On the other hand, in recent years, in order to improve an ability of controlling an inside temperature of high-temperature firing furnaces and achieving saving of energy, an aggregate of inorganic fibers subjected to needling treatment has been frequently used as a high-temperature insulting material (blanket block) by utilizing excellent properties thereof such as an extremely light weight, an easy-processing ability and a high thermal shock resistance.
However, since the inorganic fibers are usually in the form of an aggregate of fibers having various lengths, the conventional inorganic fiber molded bodies obtained merely by subjecting a slurry comprising such inorganic fibers together with a binder component, etc., to dehydration molding process and then firing the resulting dehydration-molded product tend to have such a problem that they suffer from occurrence of cracks on a surface thereof owing to mechanical shock or thermal shock upon cutting or upon use, so that the inorganic fibers and particulate matters present on the surface of the molded bodies fall off therefrom and scattered.
In particular, the above problem tends to become more remarkable in the case where the inorganic fibers include fibers having a fiber diameter of not more than 3 μm which are produced by a melt-spinning method and defined as an inhalative fiber by WHO (World Health Organization). Also, it is known that fibers obtained from a silica-based raw material are converted into cristobalite that is harmful to human body when exposed to a high temperature of not lower than 1000° C. These problems including scattering of the fibers and particulate matters and production of the harmful substances are undesirable from the standpoint of maintaining good working environments. If the molded bodies suffer from occurrence of cracks upon exposure to thermal shock or mechanical shock, the molded bodies tend to lose their function as a heat-insulating material owing to falling-off of a part of the inorganic fibers therefrom.
On the other hand, when the aggregate of inorganic fibers subjected to needling treatment are directly formed into blocks and used as a heat-insulating material for high-temperature furnaces, although the resulting material is excellent in thermal shock resistance, there tends to arise such a problem that the heat-insulating material suffers from wind erosion on a surface thereof owing to adverse influence of a wind blow speed of a burner if a high-wind blow speed burner such as a high-speed gas burner is used under some conditions, so that the fibers contained in the aggregate of inorganic fibers tend to be scattered.
To prevent scattering of the fibers, there has been proposed, for example, the technique of coating a surface of an inorganic fiber molded body with a glass layer (for example, refer to Patent Documents 1 and 2). However, in this method, there tends to arise such a problem that the glass layer is peeled off from a substrate of the inorganic fiber molded body or suffers from occurrence of cracks owing to thermal expansion when used under high-temperature conditions.
There has been proposed an alternative method in which a coating material comprising inorganic fibers, inorganic particles, an inorganic binder and an organic binder is applied on an inorganic fiber molded body (for example, refer to Patent Document 3). However, in this method, there also tends to arise such a problem that adhesion between the coating layer and the inorganic fiber molded body becomes insufficient depending upon use conditions, so that the coating layer is peeled off from the inorganic fiber molded body upon exposure to thermal shock or mechanical shock.
Further, in any of the above conventional techniques, the inorganic fiber molded body itself is produced by subjecting a slurry of fibrillated fibers to dehydration molding process. Therefore, the obtained molded products tend to be structurally poor against a load such as bending, and tend to suffer from problems such as occurrence of cracks owing to vibration caused upon processing, transportation or use as well as poor thermal shock resistance.
In addition, as the method of improving a wind erosion resistance of an inorganic fiber molded body, there have been proposed methods other than the coating methods. For example, there has been proposed the method of spraying an organic or inorganic binder onto a mat-like aggregate of inorganic fibers and subjecting the resulting product to drying and molding (for example, refer to Patent Document 4). However, this method tends to have such a problem that it is basically difficult to prepare a thick insulating material.
The above method further has such a problem that while drying the aggregate of inorganic fibers comprising the binder, the impregnated binder tends to be migrated near to the surface of the aggregate of inorganic fibers. That is, there tends to arise the problem that even when the thin mat-like aggregates of inorganic fibers obtained after drying are overlapped and laminated on each other to prepare a thick insulating material according to the above method, adhesion between the overlapped surfaces of the mat-like aggregates becomes insufficient, so that delamination of the laminated mat-like aggregates is likely to occur. In addition, since the particulate matters are more likely to be deposited on a surface of the resulting molded body rather than between the laminated mat-like aggregates, there also tends to arise such a problem that the deposited particulate matters readily fall off from the surface of the molded body upon application of mechanical shock thereto.
Apart from the above techniques, there has been proposed the method of laminating a plurality of blankets or mats formed of inorganic fibers, impregnating the resulting laminate with an inorganic binder and freezing the obtained product to produce an inorganic fiber molded body that is excellent in heat resistance and strength even inside thereof (for example, refer to Patent Document 5). However, the inorganic fiber molded body obtained by this method tends to suffer from delamination between the inorganic fiber layers and therefore tends to be remarkably deteriorated in durability against thermal shock.
To solve the above problems, there has also been proposed the method in which an inorganic fiber blanket layer is disposed an inner wall side of a furnace whereas a kneaded material layer formed of inorganic fibers, a binder and refractory particles is disposed on an outer wall side of the furnace to reduce thermal shock against the kneaded material layer and prevent propagation of cracks toward the inner layer of the furnace (for example, refer to Patent Document 6).
However, since the inorganic fiber molded article is produced from a slurry comprising an organic binder, there tends to arise such a problem that if the slurry is used in a large amount, a preliminary firing step of the molded article is required, resulting in increased costs, whereas if no preliminary firing step thereof is conducted, the obtained molded article suffers from occurrence of cracks even at a relatively low temperature.